Rigid anti-pollution requirements of international environmental protection standards have led aircraft engine designers to search for ways to reduce the pollution in turbojet engine exhaust gases. One solution is to continuously adapt the distribution of the air flow which forms the fuel-air mixture by means of mobile flaps or diaphragms to vary the inlet area of the injector air intakes as a function of the operational parameters of the engine.
U.S. Pat. No. 4,726,182 illustrates a known variable-geometry injection system. In this system, the diaphragm is supported on the outer swirler of each of the injectors and is positioned by a lever controlled by a fork supported by a shaft pivotally attached to the chamber casing. There is a separate control device passing through the casing for each of the injector diaphragms, with the synchronization of the motion of all of the injection systems achieved by a synchronizing ring located externally of the combustion chamber casing.
Although this system has effectively reduced the exhaust emissions of the engine, the large number of link rods, forks and other levers in the air flow path (which are equal to the number of injectors) interfere with the flow of cooling gases over the outer wall of the combustion chambers. The large number of control elements passing through the engine casing all require sealing to prevent excess air leakage, which renders the system unduly complex.
In other systems, the motion of the diaphragms are synchronized by a ring located between the combustion chamber casing and the outer chamber wall which interconnects the control levers of the diaphragms. This design also interferes with the flow path of the outer wall cooling gases and prevents the formation of a uniform cooling film.
French Pat. No. 2,491,140 discloses a control system incorporating an external ring seated on a roller bearing which rests on a cylindrical bearing of the combustion chamber. While this system offers the advantage of simplicity, it requires the diaphragm-driving lever to pass through the cylindrical bearing of the combustion chamber and the roller bearing, rendering it difficult to achieve a practical embodiment of this system. Also, the cooling air is poorly guided on the outer chamber wall and toward the outer swirler of the injector by the presence of an annular cavity formed by the cylindrical bearing supporting the drive ring, this cavity forming a blind hold around the injector which creates uncontrollable perturbations hampering the air guidance toward the swirler.